Polymers which include ions as an integral part of the polymeric chain are known as ionene polymers. Although a variety of structural types of ionene polymers are possible, by far the most common are polymeric quaternary ammonium salts. The first examples of such ionene polymers, as reported by Gibbs and Marvel in J. Am. Chem. Soc. 57, 1137 (1935), were prepared by reaction of low molecular weight diamines with low molecular weight alkyl dibromides, producing a polymer as a crystalline solid with a high ionic content which was insoluble in most organic solvents and very soluble in water. Numerous examples of this type of ionene polymer have subsequently been described in the scientific and patent literature, citing among other features their utility as corrosion inhibitors, wet and dry strength additives, and antimicrobial agents.
It was observed that by substantially reducing the ionic content of the ionene polymer by increasing the molecular weight of the oligomeric fragments between quaternary ammonium links with a polymer having a T.sub.g below room temperature, an elastomeric ionene polymer is produced. For example, Dolezal et al. (Rubber World, April 1968, page 46) studied the reaction of 4-6,000 MW polyisobutylene dibromides with various organic tertiary amines. The resulting ionenes were elastomeric but very weak. Strength was developed in such elastomers only after being crosslinked with multifunctional amines. Dieterich et al. have reviewed polyester polyurethane ionenes [Angew. Chem. Int. Ed. Engl., 9, 40 (1970)] and have investigated the effect of ionic content on elastomeric properties. Khojiya [Makrom. Chem. Rapid Comm. 2, 417 (1981)] discloses an ionene prepared from a polytetramethylene oxide diamine of 1000 molecular weight with 1,4-bis(chloromethyl)benzene as linking agent to produce a weak elastomer with a tensile strength of 1370 kPa (200 psi) and elongation at break of 510%. Better physical properties were reported later (IUPAC Sixth Int. Symposium on Cationic Polymerization and Related Processes. Abstract, Ghent, Belgium, Aug. 30-Sept. 2, 1983). Using 1,4-bis(chloromethyl)benzene again, but with a polytetramethylene oxide diamine of 4,400 molecular weight, a film was made with 27,000 kPa tensile strength (3900 psi) and 1000% elongation at break. However, such elastomers exhibited high permanent set when extended to 500% elongation.
A series of patents by Buckler et al. describe the quaternization of pendant tertiary amines with difunctional halides in a number of styrene-butadiene (SBR) gum rubbers U.S. Pat. No. 3,969,330; U.S Pat. No. 4,070,340), and of pendant halides in a halobutyl gum with difunctional amines (U.S. Pat. No. 3,898,253), as a means of providing improved green strength in the manufacture of tires and inner tubes. Although these elastomers are not true ionenes in the ordinary sense of the definition, the authors discovered that such crosslinks behaved as if they were labile. Under the action of mechanical shearing and heat, the Mooney viscosity of the cured elastomers was significantly redueed and the rubbers became readily millable. Furthermore, upon standing, the elastomeric properties were completely recovered and this was attributed to the reversibility of the quaternary ammonium links (Elastomerics, Dec. 1977, page 32). These results are also consistent with another, perhaps more likely explanation; namely, that milling of the cured rubber causes rupture of hydrocarbon bonds which reduces the Mooney viscosity but at the same time leaves the quaternary ammonium bonds intact. These ions would then be left as pendant groups along the polymer chains. Upon standing, these ions aggregate into clusters which act as physical crosslinks to re-establish green strength. Indeed, the authors have demonstrated that such reversible elastomeric properties can be developed without chemical crosslinking in a halobutyl rubber by simply quaternizing pendant alkyl bromides with monomeric tertiary amines (U.S. Pat. No. 4,256,857).
U.S. Pat. No. 3,904,580 appears to be the only disclosure of the preparation of an elastomer which undergoes reversible quaternization. This patent describes a styrene butadiene copolymer with pendant amines cured with 1,4-bis(chloromethyl)benzene linking agent to provide a crosslinked quaternary ammonium elastomer which is capable of reversal of the quaternization when heated to 175.degree. C. This linking agent is, however, slow to react with the pendant amines. This slowness of reactivity is a common major disadvantage in most ionene elastomer preparations. Most of the other reported preparations of true ionene elastomers often require days of heating under reflux to achieve adequate degrees of polyquaternization.